This invention relates to a battery checking device to be installed in a camera system composed of a body of a camera or a single lens reflex camera having a battery built therein and an accessory device which can be detachably mounted on the body of the camera or the single lens reflex camera and has the battery built therein.
In a single lens reflex camera of a view-finder interchangeable type, for example, there may be contemplated such a construction that a focus detection device is built into such interchangeable view-finder. In the device of such construction, it may be convenient for a photographer in his use of an ordinary view-finder (such as, for example, an eye-level view-finder), if a battery for exposure control is provided in the camera body and a battery for focus detection is provided in the view-finder.
By the separate installation of the batteries for the view-finder and the camera body, there take place two situations in the battery consumption: the one is when the battery in the camera body is used up earlier than that in the view-finder; and the other is when the battery in the view-finder is used up first.
In the former case, the focus adjusting device for the view-finder is still in an operable condition, even if the battery in the camera body is exhausted. In other words, while it is not possible to optimize the exposure, an in-focus photograph can still be taken. In the latter case, there may be produced a photograph with appropriate exposure, but an out-of-focus photograph results. This makes it necessary on the part of the photographer to constantly know whether both batteries are in their normal conditions, or not.
What has been explained in the foregoing applies not only to the relationship between the body of the single lens reflex camera and the view-finder, but also to a relationship between a camera and a flash light device as well as between a single lens reflex camera and an interchangeable lens having a focus adjusting device.